This invention relates to oil production to a floating platform, and in particular to a riser assembly for multiple flowlines. In the production of oil from subsea wells, it is required to convey the oil to the surface in order to degasify it before pumping to another location. A riser assembly for this purpose is illustrated in "Early Subsea Production System" by B. H. van Bilderbeek, page 131, the APEA Journal, 1977. This includes a central riser as a structural member and a number of peripheral flowline risers annularly spaced around the central riser. The central riser may be used for returning the processed oil to the seabed for transport to a subsea pipeline.
These flowlines are held in spaced relationship around the riser at a plurality of elevations by spiders attached to the central riser. Movement of the floating platform is accommodated by a flexible joint near the bottom of the central riser, and by flexible hoses near the bottom of the flowline risers.
A lower riser package includes the flexible joint and flexible hoses. This package is stabbed on a riser connector which releasably holds the package down and also holds the package on production flowline stab connections. Retrieval of the package or any one of the flexible pipes is accomplished by disconnecting the central riser and withdrawing the entire assembly.
Individual peripheral production risers are stabbed on the upper end of the flexible hose of the riser package with a mechanical lock and hydraulic unlock connector. Each flowline may be withdrawn after disconnecting the connector, but the flexible pipe may not be so withdrawn.
Since the riser assembly may bend in the order of 10 degrees around the flexible joint located at the bottom of the central riser, the flexible hoses must have sufficient slack to take up differential movement caused by this bending. This results in a large and, therefore, relatively small radius expansion loop, with this small radius limiting the use of TFL (through the flowline) tools.
Since the flexible pipes experience much bending, they are the most likely component to require replacement. In this prior art arrangement, it was required to remove the entire riser system and to raise the lower riser package in order to replace even one of these hoses. Not only must the entire system be withdrawn but production through all of the lines must be stopped.